1. Field of the Invention
The present invention relates to a pattern position detecting apparatus and more particularly to a pattern position detecting apparatus using a laser beam which is well suited for the recognition of the edge positions, line width, etc. of a photoresist pattern used in the fabrication process of semiconductor devices.
2. Description of the Prior Art
In the fabrication of semiconductor devices, such as ICs or LSIs, by the use of a semiconductor wafer, the wafer undergoes various treatments in the course of the fabrication process and particularly in order to fully ensure the desired characteristics it is important to accurately control the line width, dimensions, etc. of a circuit pattern or the like during the process. For this purpose, there has existed a need for an apparatus capable of automatically measuring the line width, dimensions, etc. of such pattern with a high degree of accuracy.
Various apparatus have been in actual use for measuring the dimensions, etc., of patterns on wafers and these known apparatus are mainly of the type in which the pattern image observed through an optical microscope is picked up by a television camera (ITV) and the pattern dimensions are measured in accordance with the resulting video signal. Of these conventional apparatus, those which are capable of automatic measurement suffer the following disadvantages.
(1) As the fineness of the pattern on a wafer increases, it becomes more difficult to make the measurment with greater accuracy due to the limitations imposed by the resolution of an optical microscope. This is particularly evident in the measurement of very important photoresist patterns.
(2) When measuring or detecting the pattern in the photoresist, if the kind of the base material for the photoresist layer is aluminum, polysilicon or the like, there is the danger of a situation arising in which due to the inherent material characteristic which is referred to as granulation, noise corresponding to the granulation is contained in a video signal resulting from the pattern, thus making it difficult to make the measurement.
(3) There are cases where it is difficult to make the simultaneous measurement of patterns of different kinds of materials which are present on the wafer. This may, for example, occur in cases where a relative positional deviation between a photoresist pattern formed by the exposure and other etching pattern is measured for the purpose of evaluating the alignment accuracy of an exposure apparatus or the like.
(4) When measuring the line width of a photoresist pattern, depending on the type of the photoresist, the layer thickness or the absolute dimension of the line width, the offset amount of the measured dimension value is varied, thus making it difficult to obtain an accurate dimension value.
The foregoing problems are more or less encountered in any apparatus adapted for measuring the dimensions of fine patterns and this amounts to a serious disadvantage in the case of the apparatus capable of the automatic measurement. Of course, this is not limited to the measurement of pattern dimensions and similar deficiencies are encountered in the simple detection of the small stepped edge positions of a pattern (e.g., an alignment mark for registration purposes).